An In Vitro Study of the Number of Distal Roots and Canals in mandibular First Molars in Iranian Population.

INTRODUCTION: The purpose of this study was to evaluate the number of distal roots and canals in mandibular first molars and their internal anatomy radiographically within Iranian population. MATERIALS AND METHODS: A total of 310 distal roots of mandibular first molars were incorporated in this study and evaluated in terms of number of roots and number and types of canals. Root canal systems were studied in vitro by means of radiography and based on Vertucci’s classifications. RESULTS: It was shown that 4.5% of the teeth in this study had two distal roots, of which, 100% indicated type I for both distobuccal and distolingual roots. Among all the teeth, 43.2% had two canals, 24.2% two apical foramina, and 38.7% two orifices in their distal roots. According to Vertucci’s classification 54.9% of the teeth were type I, 19% type II, 1.9% type III, 14.2% type IV, 4.2% type V, 1% type VI, 0.3% type VII and 0% type VIII. CONCLUSION: In as many as 43.2% of all teeth assessed in this study, bicanaled distal roots were observed, dentists are always recommended to search for the second canal in distal roots of mandibular first molars. In case the second canal in the distal root is missed, failure of endodontic treatment will be anticipated. A rectangular type access cavity design allows better visualization and negotiation of the probable second canals within the distal roots of mandibular first molars.


INTRODUCTION
Obviously, the main purpose of dental services is providing people with dental and oral health and its maintenance. Knowledge of internal dental anatomy plays an important role in success of endodontic treatment and negligence of this important issue leads to improper diagnosis and treatment planning. The purpose of an endodontic treatment is chemomechanical cleaning of the root canal system, maintaining its original anatomy and obturation of this space with an inert material. In order to obtain a successful endodontic treatment, dentists must be aware of the variations in root canal morphology.
According to both in vivo and in vitro studies performed on morphology of the mandibular first molars' root canal system, it has been shown that the presence of four root canals in mandibular first molars has been pointed out in most of the credited endodontic textbooks (1), but these reports are based on studies designed in European and American populations. According to Walker (2) the need for more studies in order to gain more information about the root canals and their morphology in different ethnic groups highly exists. Clearly, several studies have been done on internal anatomy in different countries, but since ethnic characteristics may influence internal anatomy e.g., more prevalent  (3), more extensive research on internal anatomy of Iranian population seem necessary. Authors have shown that internal anatomy of mandibular first molars often needs more concern because the number of roots and canals in these teeth are quite variable. On the other hand, since mandibular first molar plays an important role in mastication, knowledge of the internal anatomy of this tooth is necessary to perform a successful treatment (4). According to Skidmore (5), the mandibular first molar seems to be the tooth that most often requires endodontic treatment. The purpose of this study was radiographic evaluation of the number of distal roots, root canals and internal anatomy of the mandibular first molar teeth within Iranian population.

MATERIALS AND METHODS
In this descriptive in vitro study, a total of 310 permanent mandibular first molar teeth with intact roots were selected. These teeth were devoid of caries and/or fracture and were not endodontically treated. The age, gender and the reason for extraction were not recorded. By the time total samples were collected, all extracted teeth were rinsed and placed in 10% formalin. Then deposits, attached bones and soft tissues were removed by hand instruments and cleaned using a sonic device. After a thorough irrigation with tap water, the samples were placed and coded from 1 through 310 within plastic boxes containing normal saline. The teeth were split into mesial and distal halves by means of a diamond disc. Then the distal roots were mounted in wax and radiographs were taken from the buccolingual and mesiodistal aspects of the roots (Kodak, Ektaspeed, Eastman Kodak, NY, USA) and were exposed for 0.4s using an X-ray machine set at 70kV and 8mA. The tube-to-film distance was set at 2 cm. After all radiographs were processed, the teeth codes were recorded and the radiographs were mounted in coded frames. The radiographs were assessed by an endodontist, using an X-ray viewer and magnifying lens (×2) in a dark room. Root canal systems were evaluated and classified using Vertucci's classification (6), ( Figure 1) and variability of the distal roots of mandibular first molars were evaluated in terms of number of roots, number of canals and types of canals.

RESULTS
The results showed as follows:

DISCUSSION
In the current study, the distal roots of 310 mandibular first molars were examined radiographically. After evaluating the radiographs from buccolingual and mesiodistal aspects, the data were recorded according to Vertucci's classification. However if contrast media were injected into the root canal system, internal anatomy of distal roots would be better visualized. In this in vitro study, stainingsectioning method was performed after the radiographic assessment (7)  can give rise to more exact results because the operator can have a direct vision and a straight accessibility to the internal anatomy of the teeth but when using radiographs the operator can be mislead by the two-dimensional image of the radiograph to assess the three-dimensional statues of the tooth. One of the limitations of current study was that only one examiner interpreted the radiographs. It is better to evaluate radiographs by two or more examiners, especially in studies that radiopaque contrast media are not injected into the root canal system. It can be concluded from the data analysis of the two methods of staining-sectioning and radiographic assessment that both methods agreed upon number of roots, number of canals and types of canals based on Vertucci's classification in 100%, 87.4% and 80.3%, respectively. In mandibular first molars, additional roots can be found lingual to the main distal root and is considered more as an ethnic characteristic than a developmental anatomy. This additional root can be small-conical to fully developed shape in nature, but one must keep in mind that it contains pulpal tissue in all cases (8).
It can be inferred from the endodontic literature review concerning the frequency of mandibular first molars with two distal roots in different ethnic groups that this frequency ranges from 2.2% according to Skidmore and Bjorndal (5) to 21.5% as stated by Yew and Chan (9) (2,3,5,7,(9)(10)(11)(12)(13)(14)(15), (Table 1). It can be concluded from other investigations that the frequency of bicanaled distal roots of mandibular molars can range from 27% according to Pineda and Kattler (16) to 50% based on Wasti's study (3). The result of any of the studies varies with each other due to the techniques used, sample size and the subject population.
Comparing the results of other studies it can be understood that there is a considerable difference between Asian and European groups. For instance, within Asian groups, the frequency of bicanaled distal root of mandibular first molars vary ranging from 50% according to Wasti (3), to 80.5% according to Gulabivala (15), where as within European In studies separately evaluating distobuccal and distolingual roots, one can understand that type I configuration has the highest frequency.

CONCLUSION
Among all teeth incorporated in this study4.5% has two distal roots and 43.2% had distal roots with two canals.

IEJ -Volume 3, Number 1, Winter 2008
The dentist must always search for the second canal in the distal root. If one is missed, it can result in failure of endodontic treatment. The rectangular form of the access cavity will lead the practitioner to a better visualization and identification of the probable second canal of the distal root.